I have now managed to find a solution:-

Sub EditFont()
'To format font color for 12 digits to 4 black, 5 red, 3 black:
123456789012
'First, format digits to be treated as characters
ActiveCell.Value = "'" & ActiveCell.Value
'Format all characters for black
With ActiveCell
..Font.ColorIndex = 3
'Format characters 5 thru 12 as red
..Characters(1, ActiveCell.Characters.Count - 8).Font.ColorIndex = 1
'Reformat characters 10 thru 12 back to black
..Characters(10, ActiveCell.Characters.Count - 3).Font.ColorIndex = 1
End With